As this type of power converter, there has conventionally been proposed, for example, an electrical load device that has a DC chopper for converting a first DC voltage output from a DC power source into a second DC voltage of a voltage level different from that of the first DC voltage, and first and second electrical loads driven by the second DC voltage (see Patent Literature 1, for example).
As shown in FIG. 23, a specific configuration of this electrical load device has an AC motor M1 that drives drive wheels, and an alternator M2 that is driven by an engine and generates AC power. The AC motor M1 is driven by a voltage source inverter IV1. The AC power generated by the alternator M2 is converted into DC power by a voltage source rectifier IV2. A positive bus L1 and negative bus L2 that connect the voltage source inverter IV1 and the voltage source rectifier IV2 to each other are connected by a smoothing capacitor C. The smoothing capacitor C connecting the positive bus L1 and the negative bus L2 to each other is connected to a DC chopper CV. This DC chopper CV boosts the voltage of DC power of a DC power source B, supplies the resultant DC power between the positive bus L1 and the negative bus L2, and lowers the voltage of DC power input from the voltage source inverter IV1 and the voltage source rectifier IV2 to charge the DC power source B.
Then, the voltage source inverter IV1, the voltage source rectifier IV2, and the DC chopper CV are controlled to be driven by a controller CD.
Patent Literature 1: Japanese Patent Application Publication No. 2004-112883
In the conventional example shown in FIG. 23, however, when the alternator M2 functions as a generator, the AC power generated by this alternator M2 drives the AC motor M1. However, in the system in which the alternator M2 is driven by the engine, the AC motor M1 tends to be driven using only the DC power source as much as possible for the purpose of energy conservation or CO2 reduction.
For instance, in a hybrid vehicle using both an engine and an electric drive system, a battery or a large-volume capacitor is used as a DC power source. The battery or the capacitor with increased volume can be charged from a distribution system as well, so that the hybrid vehicle can travel approximately several tens of kilometers without operating the engine (electric vehicle (EV) traveling mode, including a regenerative braking mode). Such a plug-in hybrid vehicle is ready to be put into practical use.
During a period in which the hybrid vehicle travels in the electric vehicle (EV) traveling mode, output power of the alternator M2 functioning as an alternator is 0, and, therefore, the voltage source rectifier IV2 connected to the alternator M2 normally remains stopped. While the voltage source rectifier IV2 remains stopped, power is not supplied therefrom. Therefore, in order to obtain, from the AC motor M1, a motor torque that is equivalent to a torque obtained when the engine is operated, the power needs to be supplied from the DC power source B through the DC chopper CV. In this case, unfortunately, the problem is that it is inevitable to increase the conduction current of the DC chopper CV, increasing the size of the DC chopper CV.